The present invention relates to a method of laser marking, especially but not exclusively laser marking of hard transparent materials, and also to apparatus suitable for carrying out the method.
Permanent marking and printing of a wide range of materials is common practice, and a variety of techniques are used. Materials such as hard transparent materials including glasses have a high level of hardness and are brittle, so marking methods for these materials need to take account of these properties.
A conventional method of marking hard materials is to use a mechanical tool to write directly onto the surface of the material by surface scribing. However, a problem with using this method on glass, for example, is that many microcracks form along the edges of the scribing lines. These microcracks weaken the material and reduce the quality of the marking. Another technique uses laser ablation. At high levels of laser energy fluence (xe2x80x9cfluencexe2x80x9d being defined as laser energy delivered per irradiated area), the laser energy causes a breakdown of the hard material so that particles of the material are removed and thrown out from the material, leaving a hole or a pit. Ablation has been used to mark the surface of glass and also to mark the interior of glass bodies. However, this technique also causes microcracks around the marked areas, owing to heating and thermal stresses in the material caused by absorption of the laser energy. Marking which is free from microcracks has been demonstrated on glass by using a fluoride excimer laser for ablation, but the laser source is costly, has poor beam quality giving poor marking definition, and has a complicated configuration.
U.S. Pat. No. 6,238,847 and U.S. Pat. No. 6,075,223 propose laser-based marking methods for marking of glass, ceramic, plastics and metals. A layer of marking material is applied to the surface of a substrate, and irradiated with a laser beam in accordance with the desired pattern. The irradiated parts are heated by the irradiation and adhere or chemically bond to the surface of the substrate. The non-irradiated parts are then removed to leave the pattern on the substrate surface. However, when used to mark glass, microcracks can be formed at high laser energy levels. U.S. Pat. No. 6,238,847 suggests that the risk of cracking can be reduced if the substrate and marking material are preheated with a first laser beam before the marking material is adhered with a second laser beam.
Therefore, there is a requirement for a method of laser marking which does not cause microcracking, and/or does not require any additional steps to reduce the risk of microcracking, such as preheating, and/or does not require the application of a separate layer of marking material, and/or is simple and cost-effective.
Accordingly, a first aspect of the present invention is directed to a method of laser marking comprising:
arranging a sample of target material spaced apart from a sample of markable material;
directing irradiation having an energy fluence above the ablation threshold of the target material onto the target material such that at least some of the target material is ablated and thrown onto a surface of the markable material; and
subjecting said surface of the markable material to irradiation having an energy fluence below the ablation threshold of the markable material to induce an interaction between the ablated material and the surface which marks the surface with the ablated material.
The method permits the marking of materials with reduced or even without the formation of microcracks. The fluence of irradiation to which the markable material is subjected is below its ablation level, so the material is protected from damage. The ablation of the target material can be achieved at fluences less than the ablation threshold of the markable material, so besides protection of the markable material, the entire method can be carried out at relatively low fluence levels, which is cost-effective. Also, the ablation of the target material throws the ablated material onto the markable material surface in a localised fashion so the ablated material can be directly deposited on the surface in accordance with the desired pattern, rather than needing to be applied in a layer over the whole surface.
In a preferred embodiment, the markable material is arranged with respect to the target material such that the irradiation is directed via the markable material and onto the target material. This arrangement can be used so that the same beam of irradiation both ablates the target material and subjects the markable material surface to irradiation to mark it. The ablated material is thrown onto the back surface of the markable material (with respect to the direction of propagation of the irradiation) at the point where the irradiation passes through the markable material, so that newly incoming irradiation encounters the surface and previously ablated material thereon before reaching the target material to cause further ablation. Use of the above arrangement means that the markable material may be substantially transparent to the irradiation. The method is therefore especially suitable for transparent hard materials such as glass.
Advantageously, the irradiation is provided as a train of pulses. The markable material is hence not subjected to continuous irradiation, and any heat building up in the material from absorption of the irradiation has an opportunity to dissipate between pulses. Examples of suitable pulse durations are tens and hundreds of nanoseconds. Many sources of pulsed irradiation are available, such as Q-switched lasers, including Nd:YAG Q-switched lasers.
The ablation and the interaction may be achieved by the same pulse of irradiation, if the spacing between the target material and the markable material, and the duration of the pulse, are such that the ablated material can reach the surface of the markable material before the tail end of the pulse has passed through the markable material. Therefore, the interaction to mark the surface occurs before further ablated material is deposited. Alternatively, the ablation and interaction may be achieved by different pulses of irradiation.
The method may further comprise monitoring the fluence of the irradiation, and preferably then further comprises controlling the fluence of the irradiation in response to the monitoring. To ensure that the markable material is successfully marked without being damaged, the fluence should be below the ablation threshold for the markable material and above the ablation threshold for the target material. By monitoring and controlling the fluence, it can be maintained at a suitable level at all times.
The method may further comprise detecting and analysing the amount of irradiation reflected and scattered from the surface, the target material and the ablated material, and advantageously then further comprises adjusting the spacing between the target material and the markable material in response to the analysis of the reflected and scattered irradiation to determine that the ablated material is being thrown onto the surface of the markable material. If the spacing is too large, the ablated material has insufficient kinetic energy to reach the surface, so that marking does not occur. However, the detected irradiation can be shown to have certain identifiable features when the ablated material is reaching the surface and interacting, so detection and analysis of the irradiation can be used as an indicator that the method is being carried out successfully. Active control of the spacing in response to the detection and analysis therefore assists in keeping the method working properly.
Advantageously, the method further comprises setting the spacing between the target material and the markable material so that the amount of ablated material thrown onto the surface of the markable material is sufficient to mark the surface with a mark of a desired tone. A larger amount of ablated material gives a mark of a darker tone on the markable material. The ablated material has a spread of kinetic energies as it is thrown from the target material, and hence a spread of traversable distances, so altering the spacing changes the proportion of the ablated material which reaches the surface to contribute to the mark. This gives control over the tone.
Alternatively or additionally, the method further comprises setting the fluence of the irradiation so that the amount of target material which is ablated and thrown onto the surface of the markable material is sufficient to mark the surface with a mark of a desired tone. A higher fluence will ablate more target material and impart more kinetic energy to it, so that more ablated material reaches the markable material to contribute to the mark and increase its tone.
The method may further comprise moving the directed irradiation and the sample of markable material relative to one another so as to mark the surface of the markable material in accordance with a desired pattern.
The target material may be one of copper, silicon, aluminium, silver, chromium, titanium, tungsten and other metal, semiconductor or other solid substrates. Different materials give different colours of marking.
The irradiation may be optical irradiation, for which there are plentiful sources.
A second aspect of the present invention is directed to apparatus for laser marking a sample of markable material, comprising:
a sample of target material; and
a irradiation source operable to generate irradiation to:
ablate at least part of the target material so that the ablated material is thrown onto the surface of a sample of markable material spaced apart from the sample of target material; and
irradiate the surface of the sample of the markable material to induce an interaction between the ablated material and the surface which marks the surface with the ablated material.
This apparatus is suitable for carrying out the method according to the first aspect of the present invention.
Preferably, the apparatus further comprises a controller operable to control at least the operation of the irradiation source. Automated control of the apparatus gives faster and more accurate marking.
The apparatus may further comprise scanning apparatus controlled by the controller and operable to provide relative movement between the irradiation and the sample of markable material so that the surface can be marked in accordance with a desired pattern. Preferably, the scanning apparatus comprises a galvanometer-based beam scanner operable to scan the irradiation. A galvanometer scanner can provide fast, accurate and repeatable movement of a beam of irradiation.
The apparatus may further comprising an energy meter operable to measure the energy of the irradiation and pass the measurements to the controller, the controller being operable to control the irradiation source in response to the measurements. This allows for the energy fluence of the irradiation to be continuously controlled to ensure that it stays below the ablation threshold of the markable material and above the ablation threshold of the target material.
In a preferred embodiment, the apparatus further comprises an adjustable mount operable to adjust the spacing between the sample of target material and the sample of markable material. Control of the spacing gives control over the tone of the mark. It is also important for operation of the apparatus in that the spacing may need to be adjusted to ensure that it is small enough for ablated material to reach the markable material.
Advantageously, the apparatus further comprises a detector operable to detect irradiation scattered and reflected from the surface of the markable material, the target material and the ablated material and pass a detection signal to the controller, the controller being operable to control the adjustable mount in response to the detection signal. The signal indicates when the spacing is small enough, so the spacing can be automatically maintained at an appropriate distance.
Suitably, the irradiation source is operable to generate optical irradiation. Such sources are plentiful.